Writing over Tru-Oil will clog up a Sharpie pen, or at least when I have used them. What I do is add a quick spray of poly over the area to be written on. After that dries I write the specifics and signature and after that has dried I put another coat of Tru-Oil over that. If you spray directly over the Sharpie it will run.
Now, I use India ink to sign bows and even though it isn't affected like a Sharpie, I still follow the same technique. Pat